Field of the Invention
The present invention relates to a sound field correction technique of correcting the influence of the interference between a plurality of sound waves on the frequency characteristic in an acoustic space so as to obtain a target characteristic.
Description of the Related Art
When a sound is produced from a sound-producing source such as a speaker in a space having wall surfaces such as a wall, floor, and ceiling in a room of a house, sounds reflected by the respective surfaces of the room in addition to the direct sound reach a sound capture point in the space, and a plurality of sound waves interfere with each other. In general, the resonance phenomenon in the room mode (natural vibration mode having features such as the transmission characteristic of a room depending on the dimensions of the room) occurs at frequencies corresponding to the dimensions of the room. This phenomenon is called a standing wave. Even when no wall surface exists in a space, if a plurality of sound-producing sources are used, direct sounds may interfere with each other.
In this manner, when a plurality of sound waves interfere with each other, the interference greatly influences the frequency characteristic at a sound capture point. More specifically, when a microphone is located at the sound capture point and a measurement signal is produced from the sound-producing source to measure an impulse response between the sound-producing source and the sound capture point, peaks and dips are generated on the graph of the amplitude-frequency characteristic (dB expression of this characteristic will be called an “f characteristic” hereinafter). Especially in a low frequency band in which the influence of the room mode prevails, large peaks and dips appear on the f characteristic.
In this case, when the sound-producing source is a speaker, the sound capture point is a listening point, and the user listens to music in the room, the sound quality in audibility is greatly degraded such that the volume of a sound of a peak frequency excessively increases and causes booming, whereas a sound is omitted at a dip frequency. Therefore, a sound field correction technique of applying a filter to a reproduce signal to cancel large peaks and dips on the f characteristic of the impulse response and improve the sound quality becomes important.
FIG. 5A shows the f characteristics of a total of nine impulse responses corresponding to three sound-producing patterns (only L, only R, and L+R) between stereo speakers and three points in a listening area including a listening point in given room A. In FIG. 5A, the boundary between the low frequency band and the middle and high frequency bands is set to be 200 Hz. Especially in the low frequency band, the influence of the room mode prevails, and steep peaks and dips are generated on each f characteristic.
It is generally known that the shape of the f characteristic and the human audibility do not always coincide with each other in the low frequency band, but they match well in the middle and high frequency bands. For this reason, sound field correction is not always necessary for the middle and high frequency bands, and there is a choice of not performing correction is possible. However, sound field correction is basically necessary for the low frequency band in order to cancel steep peaks and dips. In Japanese Patent No. 3556427, when sound field correction is performed using an adaptive filter, the calculation amount is reduced by performing correction in only the low frequency band in which the f characteristic or group delay characteristic of the impulse response is disturbed.
FIG. 6 is a graph showing an example of the design of a sound field correction filter for the low frequency band of the f characteristic in FIG. 5A. An average f characteristic 601 before correction indicated by a thick dotted line is an f characteristic obtained by averaging the low frequency band portions, each as the target frequency band of sound field correction, of the nine f characteristics in FIG. 5A. The average level of the average f characteristic 601 before correction is set as a correction target level 602 indicated by a horizontal line in FIG. 6. The sound field correction filter is designed to suppress, toward the correction target level 602, steep peaks and dips on the average f characteristic 601 before correction.
For example, a biquadratic IIR (Infinite Impulse Response) peak filter capable of implementing a steep filter characteristic by a small processing amount is suitable as a filter for canceling steep peaks and dips. Peak filters that set negative and positive filter gains are assigned to respective peaks and dips on the average f characteristic 601 before correction. These peak filters are series-connected into an overall sound field correction filter. The thus-designed sound field correction filter has a correction filter f characteristic 603 indicated by a thick solid line. The correction filter f characteristic 603 is applied to the average f characteristic 601 before correction, obtaining an average f characteristic 604 after correction similarly indicated by a thick solid line. This sound field correction filter is designed not to completely raise a dip or completely lower a peak to the correction target level 602, in order to avoid excessive correction. Hence, the average f characteristic 604 after correction has a gradual undulation near the correction target level 602, but steep peaks and dips that cause a problem in audibility are canceled.
Each f characteristic in FIG. 5B is obtained by applying the correction filter f characteristic 603 to each f characteristic in FIG. 5A, and steep peaks and dips are suppressed, as in the average f characteristic 604 after correction in FIG. 6. When attention is paid to the balance of the whole f characteristic including not only the low frequency band but also the middle and high frequency bands, each f characteristic in FIG. 5B has good balance between the low frequency band and the middle and high frequency bands. More specifically, the average level of the respective f characteristics in the low frequency band is drawn as a horizontal line in the low frequency band of FIG. 5B, and the approximate straight lines (approximate characteristics) of the respective f characteristics in the middle and high frequency bands are drawn as downward sloping lines in the middle and high frequency bands. Then, at the 200-Hz boundary between the low frequency band and the middle and high frequency bands, the horizontal line of the average level of the respective f characteristics in the low frequency band and the approximate straight lines in the middle and high frequency bands are smoothly connected without a large level difference, as indicated by a circled portion. When an audition experiment was conducted in a state in which the low frequency band and middle and high frequency bands of each f characteristic were balanced, as shown in FIG. 5B, a good audibility result was obtained.
In contrast, FIG. 3A shows a total of nine impulse response f characteristics at three points in a listening area, as in room A, as for another room B different from the room for FIG. 5A. In the low frequency band of 200 Hz or lower, the influence of the room mode is weaker than that in room A, and peaks and dips on each f characteristic are not so larger than those in FIG. 5A. However, when attention is paid to the balance between the low frequency band and the middle and high frequency bands, a steep step is generated between the low frequency band and the middle and high frequency bands, as indicated by a circled portion in FIG. 3A, unlike room A. The level in the low frequency band is much higher than that in the middle and high frequency bands.
FIG. 4A shows an example of the design of a sound field correction filter for the low frequency band of the f characteristic in FIG. 3A by the same method as that described with reference to FIG. 6. A correction filter f characteristic 403 of the designed sound field correction filter is applied to an average f characteristic 401 before correction, obtaining an average f characteristic 404 after correction. The correction filter f characteristic 403 is applied to each f characteristic in FIG. 3A, obtaining each f characteristic in FIG. 3B. This f characteristic reveals that peaks and dips in the low frequency band are suppressed. However, in terms of the balance between the low frequency band and the middle and high frequency bands, the steep step between the low frequency band and the middle and high frequency bands shown in FIG. 3A still remains even in FIG. 3B after sound field correction.
When an audition experiment was conducted in a state in which the level of each f characteristic in the low frequency band was much higher than that in the middle and high frequency bands owing to a step as in FIG. 3B, the user excessively felt the low frequency band, and the audibility was greatly impaired. It is considered that even when peaks and dips in the low frequency band, which may generate a problem in audibility in general, are canceled, if the balance between the low frequency band and the middle and high frequency bands is poor, the audibility is impaired.
The method in Japanese Patent No. 3556427 cancels the disturbances of the f characteristic and group delay characteristic in the low frequency band, but does not consider the balance between the low frequency band and the middle and high frequency bands. Further, the following problem arises even in a method of introducing a filter other than the sound field correction filter in order to cancel a steep step between the low frequency band and the middle and high frequency bands, as in room B.
Ideally, a filter for canceling a steep step and adjusting the level in the low frequency band to the level in the middle and high frequency bands has a gain of 0 dB for the middle and high frequency bands and a negative gain corresponding to the step size for the low frequency band, and has a characteristic in which the gain abruptly changes at the boundary between the low frequency band and the middle and high frequency bands.
However, a great many taps are necessary to implement, by an FIR (Finite Impulse Response), a filter having a steep characteristic at a relatively low frequency. Owing to the convolution processing amount, other acoustic processes such as tone control, loudness equalization, and a compressor are hindered. If the number of taps is decreased, the characteristic becomes moderate at a portion where a steep characteristic is required, and a new peak or dip is generated at the boundary between the low frequency band and the middle and high frequency bands. For example, even when a low-shelf IIR is used, if a steep characteristic is implemented at a low frequency, the filter characteristic is disturbed at the boundary between the low frequency band and the middle and high frequency bands.
When the speaker is a multi-way speaker having a plurality of diaphragms for respective bands, the balance between the low frequency band and the middle and high frequency bands may be adjusted by adjusting the gain of a woofer in charge of the low frequency band. However, the crossover frequencies of the woofer and a squawker in charge of the middle and high frequency bands hardly coincide with the frequency of a steep step to be canceled. Even if these frequencies coincide with each other, crossover filters for band division have been applied to the woofer and the squawker. For this reason, the synthesis of the woofer and squawker after gain adjustment becomes the synthesis problem of the crossover filter having a step. A steep step corresponding to the gain adjustment amount cannot be simply implemented at the crossover frequency, and new peaks and dips are generated after all.
In this manner, when a steep step remains in the f characteristic after sound field correction, it is difficult to clearly cancel the steep step by another filter or the like. Considering that a peak filter also having a steep characteristic is used to cancel steep peaks and dips on the f characteristic in the low frequency band in sound field correction, this peak filter may also be used to cancel a steep step between the low frequency band and the middle and high frequency bands.